Solution Phase Chemistry

Characterizing and controlling the evolution of the electronic structure and the nuclear configuration during a chemical transformation represents a core objective of the chemical sciences. The direct observation of chemical transformations represents a key experimental approach to understanding chemical reactivity. The past twenty-five years has seen the understanding of reaction dynamics increase significantly with the advent of femtosecond resolution laser spectroscopy. For a limited number of molecular systems, ultrafast optical studies have provided a clear characterization of the dynamics during a chemical reaction, but for the majority of chemical systems ultrafast spectroscopy provides an incomplete and often ambiguous account of the reaction mechanism.

Distinguishing electronic and nuclear dynamics in optical spectroscopy proves difficult because both directly influence the time evolution of the spectrum. X-ray spectroscopy and scattering provide alternative approaches to characterizing electronic and nuclear structure, with distinct and complementary strengths when compared to optical methods. X-ray scattering far from an absorption edge characterizes the nuclear structure and lacks sensitivity to the valence electronic structure. Hard x-ray emission spectroscopy characterizes the electronic structure with atomic specificity and resolution largely independent of the local nuclear structure. The development of the LCLS, an ultrafast x-ray free electron laser, presents a tremendous opportunity to harness the advantages of x-ray scattering and spectroscopy to investigate chemical dynamics.

We have two complementary projects directed at solution phase chemical dynamics. These projects have been chosen to capitalize on the unique opportunity to study ultrafast chemical dynamics with a hard x-ray laser and address the distinct aspects of thermal fluctuation driven chemical dynamics and non-equilibrium photochemical dynamics. The first project focuses on the non-adiabatic dynamics of electronic excited states, in particular the dynamics of charge separation and ligand field excited state quenching of charge transfer excited states in coordination complexes. The second project focuses on non-covalent conformational dynamics, in particular the dynamics and mechanism of hydrogen bond switching, ligand exchange, and ion assembly.

We have utilized femtosecond-resolution optical pump-probe spectroscopy and ultrafast x-ray scattering to study the photo-physics and photo-chemistry of bimetallic d8-d8Ir(I)2 metal cores held together by four 1,8-diisocyano-menthane (dimen) bridging ligands ([Ir2(dimen)4]2+). The [Ir2(dimen)4]2+ complex has the dual attraction of interesting photo-physical and photo-catalytic properties. The length of the metal-metal bond in the molecular crystal depends sensitively on the steric properties of the alkyl component of the bridge and the nature of the counter ion. For dimen bridging ligands, these molecules can form weak metal-metal bonds with bond lengths as large as 4.5 Å and eclipsed 1,8-diisocyano-menthane ligands. In solution, the electronic absorption spectrum indicates that the Ir(I)2-dimer forms two bond isomers with Ir-Ir bond lengths of roughly 3.5 Å and 4.5 Å. Upon excitation to the lowest energy electronic excited state, the metal-metal bond length has been proposed to shrink significantly and only result in a single bond conformation since visible excitation promotes an electron from a metal-metal anti-bonding state to a bonding state. These large photo-induced changes in molecular structure make [Ir2(dimen)4]2+ an ideal molecule for the development of ultrafast x-ray scattering as a probe of excited state dynamics in coordination chemistry.

Schematic depiction of the time dependent spatial extent of the ligand hole generated in the LMCT excitation in [Fe(CN)6]3-. This schematic has been derived from time resolved vibrational spectroscopy measurements.

Fast and efficient energy migration and charge separation represent essential steps in molecularly based light-harvesting materials. High symmetry and strong intermolecular coupling facilitate fast energy migration, while solvent disorder leads to the symmetry breaking that facilitates charge separation. The interplay of electronic coupling and solvent disorder, both dynamic and static, has a critical impact on the electron mobility in molecular materials. We have investigated the dynamics of electron localization with polarization resolved UV pump mid-IR probe spectroscopy to investigate the dynamics of electron hole localization for excited state ligand-to-metal charge transfer (LMCT) excitation in [Fe(CN)6]3-. Our measurements showed that the initial excited state preserves the octahedral symmetry of the electronic ground state by delocalizing the ligand hole in the LMCT excited state on all six cyanide ligands. This delocalized LMCT excited state decays to a second excited state with two CN-stretch absorption bands. The presence of two CN-stretch absorption bands demonstrates that this secondary excited state has lower symmetry, which we have attributed to localization of the ligand hole on a single cyanide ligand.

Molecular structure of para-julolidinemalononitrile (JDMN) and the S1 electronic excited state potential energy surface for the torsional angles τb and τc. Rotation around c leads to a conical intersection and return to the electronic ground state, while rotation around b leads to the formation of a meta-stable twisted intra-molecular charge transfer excited state.

Mechanistic Studies of Twisted Intra-Molecular Charge Transfer

Understanding how the physical properties of an engineered reaction environment dictates the outcome of photo-isomerization reactions has wide ranging implications for designing and directing light driven chemical conversions, as well as the development of molecular sensors of local physical properties. We have used polarization resolved UV pump – mid-IR probe measurements, in conjunction with time dependent DFT calculations, to accurately measure photo-isomerazation branching ratios and rates, determine the nuclear and electronic structure of long lived excited states, and characterize the orientational flexibility of molecules in electronic excited states.

All these properties provide an excellent means of characterizing the influence of the reaction environment on photochemical dynamics with a greater level of detail than time resolved fluorescence or pump-probe measurements in the visible and UV. The combination of these measurements and TDDFT calculations has confirmed photo-isomerization generates a metastable twisted intra-molecular charge transfer excited state. The combination of bond rotation and electron redistribution highlights the dual importance of electrostatic and viscoelastic effects in photo-isomerization and sets the stage for future studies directed at determining how variations in the reaction environment influence photochemical pathways.

Aqueous ionic solutions lubricate the chemical machinery of natural and biological systems. While the unique and incompletely understood properties of water receive significant and justified attention, natural and biological processes also depend critically on the ionic species present in solution. The extent to which a dissolved ion disturbs liquid water and how water mediates ion pairing and assembly remain unresolved. We have addressed these questions by studying the interaction of water and ions from both the ion and the solvent perspective. The rearrangement of the hydrogen bonding network in aqueous solution dictates the energetics and dynamics of ion solvation. Our water-centric studies have identified the mechanism for hydrogen bond rearrangement around dissolved anions, a critical first step to explaining how water controls ionic interactions in solution. Our anion-centric studies have directly observed the dynamics and mechanism for ligand exchange around dissolved Mg2+ and Ca2+ cations and highlight the interdependence of structural flexibility and reaction mechanism.
We have used polarization-resolved multidimensional vibrational (2DIR) spectroscopy to investigate the mechanism of hydrogen bond (H-bond) exchange in aqueous 6 M sodium perchlorate (NaClO4) solutions. In computer simulations of water and aqueous ionic solutions, Laage and Hynes have shown that H-bond switching occurs via large angular jumps of roughly 60º. This description of H-bond switching conforms to intuitive expectation, given the orientational constraints imposed by H-bond formation, but contradicts the traditional view of diffusive rotational dynamics associated with the Debye model. The 2DIR measurements show that H-bonds switch via orientational jumps with an average jump angle of roughly 50º. A qualitatively similar jump exchange mechanism for H-bond switching has been observed in our experiments and the molecular dynamics simulations of aqueous 6 M NaClO4. This work provides strong experimental evidence in support of a new description of H-bond dynamics in aqueous solutions.

Schematic of (A) dissociative and (B) associative ligand exchange mechanism exemplified by thiocyanate and iodide anion exchange into and out of the first solvation shell of a Mg2+ or Ca2+ cation. Dissociative ligand exchange involves a hypo-coordinated transition state structure, while associative exchange involves a hyper-coordinated transition state structure.

The solvent-ion interaction also mediates the equilibrium and dynamics of ion recognition, pairing, and assembly in solution. My group has focused our efforts on the ligand exchange dynamics of Ca2+ and Mg2+ in aqueous solution. Ca2+ and Mg2+ have similar aqueous chemistry and similar concentrations in natural waters and blood, yet biological systems sustain intracellular Mg2+ concentration orders of magnitude higher than the Ca2+ concentration. The ion recognition mechanism utilized by cells to distinguish Ca2+ and Mg2+ remains unclear, but the mechanism for ligand exchange into and out of the ion first solvation shell likely plays a key role.

We have used the CN-stretch of thiocyanate to investigate the mechanism and dynamics of ligand exchange for aqueous Ca2+ and Mg2+ with 2DIR spectroscopy. These studies have shown that ligand exchange follows a dissociative exchange mechanism for Mg2+ and an associative mechanism for Ca2+. These measurements indicate that the lability of the first solvation shell dictates the ligand exchange mechanism, where a narrow distribution of equilibrium solvation shell structures supports dissociative ligand exchange and a broad distribution of equilibrium structures leads to associative ligand exchange. These mechanistic distinctions could be a key to the biological recognition of Ca2+ and Mg2+.